Field of the Invention
This invention relates to an acoustic wave diagnostic apparatus and to a method of controlling such apparatus.
Description of the Related Art
With an ultrasound diagnostic apparatus, a tomographic image of a specimen is obtained by transmitting ultrasound waves toward the specimen and making use of an ultrasound echo signal that represents an ultrasound echo from the specimen. With a pulsed-Doppler ultrasound diagnostic apparatus that employs a resonating-type switching power supply, there is the danger of misdiagnosis owing to the mixing of noise with a blood flowrate pattern, which is displayed on a screen, due to the switching frequency and resonant frequency. For this reason, there are instances where control is exercised in such a manner that whole-number multiples of the switching frequency and resonant frequency of the power supply fall outside the shift region of the Doppler shift frequency (Prior Art Document 1). Further, there is an arrangement in which pulse-width control is utilized in backlighting in an ultrasound diagnostic apparatus (Prior Art Document 2).
Patent Document 1: Japanese Patent Application Laid-Open No. 2007-29198
Patent Document 2: Japanese Patent Application Laid-Open No. 2008-191393
FIGS. 16 and 17 are examples of a display screen of an ultrasound diagnostic apparatus for measuring the velocity of a blood flow or the like. The velocity scale in both FIGS. 16 and 17 is represented by Doppler shift frequency. Since there is a fixed relationship between velocity and Doppler shift frequency, there are also cases where the velocity scale is represented by velocity as well as cases where it is represented by frequency. FIG. 16 illustrates noise 120, which is produced on a display screen 60 when the pulse frequency used in pulse-width control is 200 Hz, in a case where the boundary value of a velocity scale 120A to be set has been set to 2.3 kHz and, moreover, backlighting of the display device is controlled using pulse-width control. The six waveforms extending along the horizontal direction are noise. Thus, the noise 120, which has a frequency that is a whole-number multiple of 200 Hz utilized as the pulse frequency, is produced. FIG. 17 illustrates noise 121 produced on the display screen 60 in a case where the boundary value of the velocity scale 120A to be set has been set to 41.7 kHz and the pulse frequency used in pulse-width control is 20 kHz. Thus, when backlighting of the display device is controlled utilizing pulse-width control, noise 120 or 121 appears on the display screen 60 and this can bring about misdiagnosis. Whereas the Doppler shift frequency used in diagnosis generally is on the order of 100 Hz to 50 kHz, the frequency of pulses used in backlight control is on the order of 200 to 20 kHz. Since both frequencies happen to overlap, the above-mentioned noise appears on the display screen. In particular, since a high sensitivity is required in a continuous-wave Doppler ultrasound diagnostic apparatus used to detect weak-power backflow, noise is even more conspicuous. In both Prior Art Documents 1 and 2, absolutely no consideration is given to elimination of the effects of noise thus produced by pulse-width control.